Add Mattermost satellite

TODO:

• Finalize docs
• Add how-to guide in operator docs for how to set up
• Figure out how to handle errors in listener (#210 (closed))
• Move FSM changes to own MR, handle thread-safety (!683 (merged))
• Decide on @channel for warning / critical
• Check if curl wrap is needed for flatpak, adjust build images (build-images!39 (merged))
• Rename to just Mattermost?
• Disable werror for openssl, maybe pass -W0?
• Add ignore_fsm_status option
• Figure out cpr in clang-tidy job